An updated classification of the anatomical variations of the internal iliac venous drainage system: Surgical implications for anterior lumbar spinal approaches

Background: Advanced pelvic surgery is associated with potential vascular risks. The aim of this study was to complete the existing classification of the anatomical variations of the internal iliac veins encountered on a series of preoperative angio CT with a view to performing anterior lumbar spine surgery. Materials and Methods: In this monocentric retrospective study conducted between 2010 and 2020, all preoperative angio CT performed before an anterior lumbar surgery were systematically analyzed. All the abnormalities of the iliac veins were referenced in an updated classification system. Results: 910 patients (431 men and 479 women) with a mean age of 49 years [16-88] were included. Apart from the most common variant in the population (type I), 64 anatomical variations (7.0%) in the iliac veins were reported and classified according to our new classification. The percentage of coverage of the L4-L5 intervertebral disc is 52%, including 32% by the inferior vena cava before the confluence of the common iliac veins. At the level of the L5-S1 intervertebral disc, the coverage is 30% (same distribution between left and right). Conclusions: Variations of the iliac veins are frequent, and contrary to what one might think, and even if they can represent an anatomical trap during surgery, certain variations do not limit anterior lumbar spine surgery and are not more associated with vascular complications. Nevertheless, these anatomical variations must be known before any advanced pelvic surgery. Depending on their distribution, level L5-S1 is more suitable for ALIF, level L4-L5 for OLIF approaches.